This article gives tips on bulk importing users in Network Manager.

1. There is an option to fill in gaps in user numbers. Doing so results in all imported users having the same user number. To recover from this, uncheck this option and import again.
2. User numbers can be constrained. By default, Mac OS X Server uses numbers from 100 to 32767 but this range can be changed when importing users. The problem occurs when the range is smaller than the number of users being imported. In this case, when the maximum number is reached, the numbers will wrap and users will be imported with duplicate user numbers. To recover, adjust the range to the correct size and import again.
3. All imported users have login disabled. To correct this, select all the users and click the Uncheck the Login Disabled button.